ISBN 9783868538755

Zusammenfassung / Abstract

Water and its solid form - ice - is a fascinating system. It may look simple but water´s puzzling properties are still not understood. An understanding of the numerous anomalies of water is closely linked to an understanding of the phase diagram of the metastable non-crystalline states of ice. The process of pressure induced amorphization of crystalline ice has been investigated first by Mishima et al. by pressurizing hexagonal ice Ih at 77 K to above 1.0 GPa to produce high-density amorphous ice (HDA). The authors also coined the term polyamorphism, which has been observed for the first time in amorphous ice and is the term for having multiple distinct glassy states. Three distinct structural states of amorphous water are known, which can be distinguished by means of their densities at 77 K and 1 bar. They are called low- (LDA), high- (HDA) and very high-density amorphous ice (VHDA).

In principle amorphous states show a non-equilibrium nature. However, an equilibrium metastable with respect to crystallization can be reached, and transitions between amorphous states at sufficiently high temperature may be very much alike equilibrium phase transitions.

This thesis shows from isothermal compression and decompression measurements in the vicinity of the crystallization limit that the LDA/HDA transition resembles a first-order phase transition, whereas the HDA/VHDA transition resembles a continuous, higher-order phase transition. A new preparation procedure leads to a relaxed form of high density amorphous ice (e-HDA ), which has a much higher thermal stability compared to HDA studied so far.